A masked read-only memory device (hereinafter referred to as mask ROM device) is a information storage device into which information has been written with use of a particular photomask during fabrication of the memory device. Once information is thus written into the mask ROM device, the information is retained throughout use of the memory device. Such a memory device is useful especially for fixedly storing any program typically in a microcomputer system.
Such a semiconductor memory device has a drawback in that the addresses of the individual memory cells of the device have already been fixedly determined so that the address space for the memory device could not be modified arbitrarily. It is thus impossible to revise the information which has been written into a mask ROM device (as by re-numbering the address pages). It is further pointed out that, for a mask ROM device used for the storage of Japanese letters and characters, approximately two thirds of the total address space available for the memory device is left unused.
It is, accordingly, an important object of the present invention to provide a semiconductor memory system which is capable of arbitrarily revising the information which has been written into a main memory system using a mask ROM device.
It is another important object of the present invention to provide a semiconductor memory system in which the addresses for the memory array of the system are readily programmable independently of those which have been fixed for the memory array of a main memory system.
It is still another important object of the present invention to provide a semiconductor memory system which will make it possible to utilize the unused portion of the address space available for the storage of information in a main memory system.